An orbital shaker mechanism is a mixing or stirring device used especially in scientific applications for mixing or stirring containers, such as beakers and flasks holding various liquids on a platform. Specifically, an orbital shaker translates a platform in a manner such that all points on the upper surface, in the X-Y plane, of the platform move in a circular path having a common radius. Generally, beakers, flasks, and other vessels are attached to the upper surface of the platform such that the liquid contained therein is swirled around the interior side walls of the vessel to increase mixing and increase interaction or exchange between the liquid and local gaseous environment. Conventionally, the mechanism which drives the platform in an orbital translation includes one or more vertical shafts driven by a motor with an offset or crank on the upper end of an uppermost shaft such that the axis of the upper shaft moves in a circle with a radius determined by the offset in the shaft, i.e., by the "crank throw". The upper shaft or shafts are connected to the underside of the platform via a bearing to disconnect the rotational movement between the upper shaft or shafts and the platform. On multishaft mechanisms, rotation of the platform is generally prevented by a four-bar-link arrangement of the shafts. On single shaft mechanisms, the rotation of the platform is generally prevented by connecting an additional linkage between the platform and base.
In operation, the mass of the shaft above the offset or crank throw, the platform with its mounting hardware and the load consisting of the filled flasks or vessels, and the clips or fasteners which hold the vessels to the platform all translate at the rotational velocity of the driven shaft in a circle with a radius equal to the crank throw. The mass of the liquid within the vessels translates at the shaft rotational velocity in a circle with a radius equal to the crank throw plus the distance from the center of the vessel to the center of mass of the liquid contained in the vessel.
The forces resulting from the total orbitally rotating mass can often cause motion of the base of the shaker which can superimpose additional motion components into the liquid in the vessels and lead to undesirable turbulence or splashing. These forces can also cause the base unit to move or "walk" along its support surface.
In prior attempts to "balance" these destabilizing forces and thereby reduce undesired motion of the shaker, various two plane counterbalancing techniques have been proposed. Typically, the counterbalance consists of a counterweight which rotates at the shaft rotational velocity while being located in an offset position opposite to the direction of the shaft offset or crank throw. The result of this is that, in the X-Y plane, the forces generated by the translation of the platform and load are countered or "cancelled" by the forces generated by the counterweight. Unfortunately, for the destabilizing forces to be fully cancelled, the counterweight would need to be located in the same plane, i.e., with respect to the Z axis, as the centroid of the combined mass of the platform and load. This, however, is not a practical or acceptable arrangement and, therefore, in a typical platform type shaker device the counterweight is mounted below the platform in a second plane.
The Z-axis disparity results in a rotating moment being applied to the shaft along the X-Z axis. This moment transfers force through shaft bearings to the base, resulting in each foot or base support member being alternately loaded and unloaded once per revolution in a phase relationship relative to the translation of the platform and load. For this reason, the force generated by the X-Z moment often still results in undesirable splashing or turbulence of the liquid within the vessels and "walking" of the shaker unit.
In view of the above-noted deficiencies, it would be desirable to provide a counterbalancing mechanism for an orbital shaker apparatus which greatly reduces the X-Z axis moment and therefore improves the stability of the apparatus and reduces splashing or turbulence of the liquid within the vessels during operation.